Polishing slurry for ionic materials, method for selecting a dispersant to be contained in the polishing slurring, method for determining a mixing concentration of the selected dispersant and polishing method using said polishing slurry

ABSTRACT

A polishing slurry is disclosed, which is to be used for polishing an ionic material, the polishing slurry including a dispersant which is to form a nonionic adsorbing layer on a surface of the ionic material. The dispersant may be selected by separately preparing first and second solutions containing first and second different dispersants, immersing test pieces each made of said ionic material into the first and second solutions, respectively, comparing a step between an etched portion and a non-etched portion of the test piece immersed in the first solution with a step between an etched portion and a non-etched portion of the test piece immersed in the second solution, and selecting the dispersant used in the solution in which the test piece having the smaller step is immersed.

PRIORITY CLAIMED

The convention priority of Japanese Patent Application No. 2004-370,958filed on Dec. 22, 2004 is claimed in this U.S. application, the contentsthereof being entirely incorporated hereinto by reference.

BACKGROUND OF THE INVENTION

1. [Field of the Invention]

The present invention relates to polishing of ionic materials, and moreparticularly the invention relates to a polishing slurry suitable forobtaining a polished ionic material with a high-quality mirror surfacehaving super smoothness and less surface defects, a method for selectinga dispersant to be contained in the polishing slurry, a method fordetermining a mixing concentration of the selected dispersant and apolishing method using said polishing slurry.

More specifically, the invention relates to a technique favorably usableto finish surfaces of materials for deep ultraviolet-range opticallenses and fluoride crystalline materials such as a CaF₂ material, whichattach great importance to the surface smoothness and less surfacedefects.

2. [Background Art]

Various polishing slurries have been known as polishing slurries to beused for ionic materials such as the calcium fluoride (CaF₂) material asan optical crystal.

There is a polishing slurry using cerium oxide as such an example (Forexample, JP-A 2003-503223, pages 2 to 37 and FIG. 4). This publicationmentions that the cerium oxide polishing composition is used in a finishpolishing step or its polishing prestage step. This publication alsodescribes that colloidal silica, colloidal alumina, colloidal zirconiumdioxide, colloidal diamond, etc. are used in finish polishing. It alsodescribes that the pH of the polishing slurry is set to from pH2 to pH2.Further, it discloses that a polishing composition-fixed pad containingfine particles thereof is used. According to the above-recited polishingmaterials, the well known technique to be used for polishing the opticallenses is applied to the polishing of the optical lenses and performs offluoride crystals to be finely lithographed.

Another example is a polishing composition using low-viscosity siliconeoil (For example, see JP-A 2004-98242, pages 2 to 6, FIG. 2). Thispolishing composition is aimed at suppressing the formation of a roughedsurface in polishing an article made of the crystalline material offluoride such as CaF₂ through a reaction between the article and anaqueous polishing liquid when the article is to be polished. Accordingto JP-A 2004-98242, the roughened surface to be produced due to thereaction between the article and the polishing liquid can be suppressedby using the low-viscosity silicone oil as an non-aqueous polishingliquid. Further, JP-A 8-19943 discloses another method to prevent areaction between an article and a polishing liquid. According to thismethod, when the article is to be polished, fine powder of a crystallinematerial constituting the article to be polished is preliminarily addedto the polishing liquid in such an addition amount of 50% or more of asaturated dissolved amount. This can prevent the reaction between thepolishing liquid and the crystalline material, so that pits, surfacescratch, burning can be prevented.

Problems to Be Solved by the Invention

In general, when the fluoride-based crystalline material reacts with theaqueous polishing liquid, surface roughness is worsened, surface defectsincrease and the polished profile changes. When a laser interferometeris used for example, this change can be observed as a change frominterference fringes linearly formed at an equal interval to anothertype of interference fringes formed with ridge lines representingcrystalline orientations. The laser interferometer is to macroscopicallyevaluate the profile of a surface of a sample. A scanning typeinterference microscope is to evaluate a microstructure such as steps,pits and the like on the surface of the sample.

FIGS. 1(A) and 1(B) show an example of such a change. FIG. 1(A) shows asurface image of a change in surface profile of a CaF₂ single crystallens 1 immediately after being polished, the surface image beingobtained by the laser interferometer. FIG. 1(A) shows change ininterference pattern from State 1 a immediately after a lens waspolished to State 1 b in which a polished shape was changed by areaction between an aqueous polishing liquid. FIG. 1(B) is a 3D image ofthe State 1 b in which the polished shape was changed. The lens 1 waspolished with a CeO₂ slurry at near pH 10 in which an anionic dispersant(polyacrylic acid salt) was added. This CeO₂ slurry is generally usedfor polishing the optical glass lenses.

The optical axis of the lens 1 is in a [111] axial direction, andtriangular pyramid-shaped ridge lines 2 are formed on the surface due tothe crystalline orientation. The ridge line 2 is formed in such a sizeas being clearly discernible. This phenomenon is more likely to occurwhen the aqueous polishing slurry is used as the polishing compositionas compared with another type polishing composition. The occurrenceprobability of this phenomenon becomes greater in case that the anionicslurry is used as the polishing composition. From the above, it isconsidered that this is caused by the phenomenon that a chemicallyremoving action does not proceed uniformly on the lens surface, andetching occurs depending upon crystal anisotropy in which the reactionspeed differs among crystalline orientations. The reason why the anionicdispersants are frequently used is that many of them are relativelysafety and the anionic dispersants attain strong dispersability due tothe electrostatic repulsion. However, when the anionic dispersant isused, there is a limit of around 0.3 nm upon the surface roughness interms of the self average root roughness (rms value). This isattributable to the fact that the chemically removing action with theaqueous polishing liquid does not lead to reduction in surface roughnessin the case of the fluoride crystal.

FIG. 2 is a surface image of a CaF₂ single crystal lens 3 after beingpolished in a method different from that in FIGS. 1(A) and 1(B). Thelens 3 was polished with a slurry of diamond dispersed in pure wateronly. On the surface of the lens 3 are scattered shallow scratch-likedefects 4 having depths of around subnanometers. Such minute defects 4can be observed not with a Normalsky microscope having a verticalresolution of around submicrometer but clearly with a scanning typeinterference micrometer using white light. It cannot be denied that evensuch minute defects 4 can affect the span life of the lens in the deepultraviolet range. According to the polishing with the diamond slurry,the surface of the lens is predominantly mechanically removed with finediamond particles, and the surface roughness at the rms value can becontrolled to 0.2 nm or less. However, since no particular dispersant isadded, the abrasive grains are likely to be aggregated and defects 4such as scratches are likely to occur. Further, it cannot be said thatcompletely no chemically removing action occurs even near a pH neutralarea in which the shape is relatively hardly changed to the trigonalpyramid fashion. Therefore, the defects 4 are considered to be latentdamages formed by etching.

FIGS. 1(A) and 1(B) and FIG. 2 show examples of the lenses etched withthe polishing slurries described in JP-A 2003-503223. In this way, itcannot be said that the polishing slurries, etc. described in JP-A2003-503223 exhibit particularly excellent polishing characteristics forionic materials called alkali halides (halides of alkali earth elements)including fluoride crystals.

On the other hand, JP-A 2004-98242 discloses that the low-viscositysilicone oil is used as a polishing liquid so as to prevent the surfaceof the ionic material called alkali halide (halide of alkali earthelement) such as fluoride crystal from being roughened through areaction between the ionic material and the polishing liquid. However,when the low-viscosity silicone oil is used, it is more difficult towash the ionic material after polishing, as compared with use of thewater-soluble polishing slurry. For this reason, the aqueous polishingslurry is preferably used. In addition, the water-soluble polishingslurry has an advantage that the water-soluble polishing slurry iseasily prepared when pure water is used as a solvent.

In order to retard the reaction in which the material is dissolved intothe polishing liquid, a fine powder of a crystalline materialconstituting the article to be polished is preliminarily incorporatedinto the polishing liquid. However, fine powders must be prepared forcorresponding fluoride materials constituting articles to be polished.

Summary of the Invention

Under the circumstances, an object of the present invention is toprovide a polishing slurry which is used for polishing the ionicmaterials and affords excellent polished characteristics such asreduction in surface roughness and surface defects.

Further, it is another object of the present invention to provide amethod for selecting a dispersant suitable for the polishing slurry tobe used for polishing the ionic material and a method for determining amixing concentration of the dispersant.

Furthermore, it is a further object of the present invention to providea polishing method which affords excellent polished characteristics uponthe ionic material.

Countermeasure to Solve the Problems

In order to solve the above problems, the polishing slurry according tothe present invention is to be used for polishing an ionic material,said polishing slurry comprising a dispersant which is to form anonionic adsorbing layer on a surface of the ionic material.

The polishing slurry according to the present invention may comprisepure water (dispersion medium), diamond powder and the dispersant. Asthe ionic material to which the polishing slurry according to thepresent invention can be applied, CaF₂, LiF, MgF₂ and BaF₂ may berecited.

According to the polishing slurry of the present invention, the nonionicadsorbing layer is formed on the surface of the ionic material with thedispersant contained in the polishing slurry. The nonionic adsorbinglayer prevents a reaction between the ionic material and the polishingliquid and occurrence of etching. For this reason, when the ionicmaterial is polished with the polishing slurry of the present invention,it is possible to obtain an excellent polished shape and excellentpolished properties in which surface roughness and surface defects arereduced.

The dispersant-selecting method according to the present invention,which is adapted to select the dispersant to be incorporated into theclaimed polishing slurry, comprises separately preparing a firstsolution containing a first dispersant and a second solution containinga second dispersant different from the first one, immersing test pieceseach made of said ionic material into the first and second solutions,respectively, while a portion of each of the test pieces is masked ornot immersed, comparing a step between an etched portion and anon-etched portion of the test piece immersed in the first solution witha step between an etched portion and a non-etched portion of the testpiece immersed in the second solution, and selecting the dispersant usedin the solution in which the test piece having the smaller step isimmersed.

According to the above dispersant-selecting method, since the dispersantis selected by comparing the step formed on the test piece for the firstsolution with that formed on the test piece for the second solution, thedispersant can be selected, which can effectively prevent the reactionbetween the ionic material and the polishing liquid.

The dispersant-selecting method according to the present invention,which is adapted to select the dispersant to be incorporated into theclaimed polishing slurry, comprises separately preparing a firstsolution containing a first dispersant and a second solution containinga second dispersant different from the first one, immersing test pieceseach made of said ionic material into the first and second solutions,respectively, while a portion of each of the test pieces is masked ornot immersed, comparing an average size of pits formed on the test pieceimmersed in the first solution by etching with that of pits formed onthe test piece immersed in the second solution by etching, and selectingthe dispersant used in the solution in which the test piece having thesmaller average pit size is immersed. The average size of the pits aredetermined as follows. That is, the pits each have almost an equilateraltriangle for one polishing slurry. With respect to one pit, lengths ofthe three lateral sides are measured, and such measurements arecontinued with respect to other pits until statistical data giving astatistically significant difference are obtained (for example, 10 pitsgive 30 statistical data), and the average size of the pits is obtainedby averaging the sum of the statistical data (30 data) by 30. Withrespect to another polishing solution, such an average size of the pitsis determined.

According to the above dispersant-selecting method, the size of the pitis regarded as the step formed by etching. Since the dimension of thepit can be observed with a general optical microscope, the dispersantcan be more easily selected as compared with a case where the step needsto be observed with a scanning type interference microscope.

A method for determining a mixing concentration of the dispersantaccording to the present invention, which sets the mixing concentrationof the above-selected dispersant to be incorporated into the claimedpolishing slurry, comprises preparing a plurality of solutions havingdifferent concentrations of the dispersant, respectively, immersing testpieces made of an ionic material into the plurality of the solutions,respectively, while a portion of each of the test pieces is masked ornot immersed, determining a relation between a step between an etchedportion and a non-etched portion of each of the test pieces immersed inthe plurality of the solutions, respectively, and the concentration of acorresponding one of the solutions, and setting the mixing concentrationof the dispersant to be used in actual polishing based on the thusdetermined relation.

According to the above mixing concentration-setting method, the mixingconcentration to be used in actual polishing is set based on therelation between the steps formed through etching and the concentrationof the solutions. Therefore, the mixing concentration which is suitablefor preventing or suppressing the etching due to the reaction betweenthe ionic material and the polishing slurry can be set.

According to the polishing slurry of the present invention, a nonionicwater-soluble synthetic polymer is preferably added thereto as the abovedispersant.

According to this polishing slurry, the nonionic adsorbing layer can beformed by incorporating the nonionic water-soluble synthetic polymerinto the slurry.

The polishing method of the present invention comprises polishing anionic material with the polishing slurry as mentioned above using thedispersant selected by the above selecting method at a mixingconcentration determined by the above mixing concentration-determiningmethod.

According to the above polishing method, an defect-free optical elementwhich is suitable in a deep ultraviolet range and has high precision andexcellent surface smoothness can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, reference is made to theattached drawings, wherein:

FIGS. (A) and 1(B) show changes in surface profile of a lens made of aCaF₂ single crystal immediately after polishing with a CeO₂ slurry, FIG.1(A) being a surface image obtained at a measuring wavelength of 632.8nm with a laser interferometer, and FIG. 1(B) being a three-dimensionalimage obtained from FIG. 1(A).

FIG. 2 is a surface image obtained by a photographic mode of a scanningtype interferometer (trade name: NewView 5000), showing changes in thesurface profile of the CaF₂ single crystal lens polished with a diamondslurry.

FIG. 3 is a schematic view illustrating an etching experiment with useof a test piece of the CaF₂ single crystal.

FIG. 4(A) is a surface image of a test piece obtained with a laserinterferometer, showing a small step B formed on an etched surface ofthe test piece, and FIG. 4(B) is an enlarged three-dimensional imageobtained by the scanning type interference microscope.

FIG. 5 is a graph showing minute steps B formed on respective testpieces.

FIGS. 6(A) to 6(D) show surface images of non-etched surface(step-standard surface) and etched surfaces of respective test piecesplaced in respective solutions.

FIGS. 7(A) and 7(B) are a graph for showing the relationship between theminute steps of the CaF₂ single crystal formed through etching and theconcentrations of sodium CMC and a graph for showing a linear regressionthereof, respectively.

FIGS. 8(A) and 8(B) are schematic views showing a polishing apparatusand a surface structure of a polishing tool, respectively.

FIG. 9 is a surface image showing changes in surface profile of asemi-spherical lens of a CaF₂ single crystal immediately after polishingwith a diamond slurry added with sodium CMC.

FIG. 10 is a surface image of the spherical lens of the CaF₂ singlecrystal polished with the sodium CMC-added diamond slurry in aphotographic mode by the scanning type interference microscope (tradename: NewView 5000), showing a surface profile of the lens.

DETAILED DESCRIPTION OF THE INVENTION

The method for selecting the nonionic water-soluble synthetic polymer tobe added to the polishing slurry according to the present invention willbe explained. FIGS. 3(A) and 3(B) are schematic views illustrating anetching experiment used for this selecting method.

First, plural water vessels 10 and plural test pieces 11 are prepared.Into the water vessels 10 are poured solutions having different additivedissolved therein. In the present embodiment, three kinds of thesolution were prepared, that is, (1) 1-liter pure water (pH7), (2)1-liter pure water in which 100 mg of a salt of a polyacrylic acid(sodium polyacrylate or the like) is added and dissolved, and (3)1-liter pure water in which 100 mg of sodium carboxylmethyl cellulose isadded and dissolved (near pH 7). Each of these water-soluble syntheticpolymer also functions as a dispersant for the grains in the polishingslurry.

Each of the test pieces 11 is made of a CaF₂ single crystal, andmirror-finished. Each of the test pieces 11 has a face (111) for etchingwhich is partially covered with a mask 11 a. The mask 11 a is made of anaggregated film of a solubilized pitch (a pitch for polishing opticallenses). In the case of the test piece made of the CaF₂ single crystal,1 g or more of sodium CMC may be used for 1-liter pure water.

The test pieces 11 are immersed in the respective water vessel 10, andeach of the solutions is stirred always at 200 rpm with a stirrer 12.This state is kept for 48 hours as it is so as to etch the test pieces11.

After 48 hours pass, the test pieces 11 are taken out of the respectivesolutions, and their masks 11 a are removed with a solvent. No etchingis performed at a portion 13 of the test piece covered with the mask 11a(See FIG. 6(A)). For this reason, the etching-proceeded degree of eachof the test pieces 11 can be observed based on the covered portion 13 asa reference. FIG. 4 shows a minute step B formed on the etched face ofthe test piece 11 between the covered portion 13 and the etched portion14 in which etching proceeds. The minute step B is observed with ascanning type interference micrometer using white light. The etchedsurface was also observed with a microscope or the like.

As a result, the etching-proceeded degree of the test piece 11 in thesolution of the polyacrylic acid salt is three time as much as that inthe pure water. On the other hand, the etching-proceeded degree of thetest piece 11 in the sodium CMC solution was a half of that in the purewater. The above comparisons revealed that the sodium CMC prevents orsuppresses the reaction between the CaF₂ single crystal and thepolishing liquid. In this way, it is possible to select the nonionicwater-soluble synthetic polymer which can prevent or suppress thereaction between the CaF₂ single crystal as an ionic material such as afluoride crystal and the polishing slurry.

Further, the water-soluble synthetic polymer can be selected byobserving pits P (minute depressions) formed on each of the test piecesinstead of comparing the etching-proceeded degrees. FIG. 6(A) shows asurface image of the covered portion 13 for each of the test pieces 11.FIGS. 6(B) to 6(D) show surface images of the etched portions 14 b, 14 cand 14 d of the test pieces 11 in the solutions (1) to (3),respectively. When the face (111) of the CaF₂ single crystal was etched,triangular pyramids P and latent scratches were formed. The dimension (alength of a side) of the pit P is proportional to the etching-proceededdegree of the test piece 11. Since the dimensions of the pits P are froma few μm to around 20 μm, so that they can be observed with an ordinaryoptical microscope. From this, even if there is unavailable ahigh-precision apparatus which can measure the minute steps B, thewater-soluble synthetic polymer capable of preventing or suppressing thereaction between the CaF₂ single crystal and the polishing slurry can beselected by utilizing the dimensions of the pits P instead of the minutesteps B.

For example, JP-B 2820328(high-speed finish-polishing agent) in the nameof SUN TOOL adapts the construction in which a water-soluble syntheticpolymer such as sodium CMC is added to the polishing slurry. However,the object of this publication differs from that of the presentinvention in that the polymer is added to impart viscosity upon thepolishing slurry.

Next, how to set the mixing concentration of sodium CMC thus selectedwill be explained.

Similarly to the etching experiments, plural water vessels 10 and pluraltest pieces 11 are prepared (See FIG. 3). Solutions of different amountsof sodium CMC each dissolved in 1-liter pure water, respectively, areprepared, and poured into the water vessels 10, respectively. Each ofthe test pieces 11 is immersed into the respective one water vessel 10,and the minute step B formed on the etched face (See FIG. 4) isobserved.

As shown in FIG. 7(A), the dimension of the minute step B graduallydecreases as the addition amount of the sodium CMC is increased. Whenthe concentration of sodium CMC is expressed by logarithm, linearregression is possible between the concentration of sodium CMC and theminute step B (See FIG. 7(B)). From this linear regression, when theaddition amount of sodium CMC per 1-liter pure water was 1400 mg, thedimension of the minute step B was ¼of that in the case of pure water.In this way, it is possible to set an appropriate mixing concentrationof the sodium CMC from the correlation between the concentration ofsodium CMC and the etched amount.

A diamond slurry was prepared by adding and dissolving 1400 mg of thesodium CMC selected by the above selection method into 1-liter purewater so that the mixing concentration thereof may be that set by theabove mixing concentration-setting method. A diamond powder to be usedfor this purpose was used in an amount of 2 g, while its grain sizedistribution was not more than 0.2 μm. A lens R of the CaF₂ singlecrystal is polished with this diamond slurry (See FIGS. 8(A) and 8(B)).

FIG. 8(A) shows a polishing apparatus 16 equipped with a known polishingtool 15. The polishing tool 15 has a solubilized pitch-aggregated filmin a thickness of not more than 0.3 mm on a semi-spherical substratehaving grooves. The polishing apparatus 16 comprises a turntable 17, anouter vessel 18, an inner vessel 19, a reciprocating plate 20, a linearguide 21, a load 22, and a stick pin 23. The outer vessel 18 is placedon the turntable 17. The inner vessel 19 is placed inside the outervessel 18. The polishing tool 15 is arranged inside the inner vessel 19.The reciprocating plate 20 is arranged in parallel and spaced from theturntable 17. The reciprocating plate 20 is provided with the linearguide 21, the load 22 and the stick pin 23. The lens R of the CaF₂single crystal is arranged at a tip of the stick pin 23. The CaF₂ singlecrystal lens R faces the polishing tool 15 inside the inner vessel 19.Water at a constant temperature is circulated in the outer vessel 18.The above-mentioned diamond slurry is poured into the inner vessel 19.

FIG. 9 shows changes in the surface profile of the CaF₂ single crystallens R immediately after being polished with the diamond slurry by thepolishing apparatus 16. FIG. 9 gives the surface images obtained by alaser interferometer. The interference fringes change from State Raimmediately after polishing, State b to State c. FIG. 10 shows thesurface image of the lens R of the CeO₂ single crystal. As shown in FIG.9, the surface profile of the CaF₂ single crystal lens R did not changeto triangular pyramid pattern. If a medium diameter size (40 to 70 mm indiameter) is taken for the lens R, the sphericity: λ/30 to λ/50 could beobtained. The surface roughness (rms value) could be attained at notmore than 0.2 nm. As shown in FIG. 10, the surface of the lens wasalmost free from defects. The reason for this is considered that anonionic adsorbing layer was formed on the surface of the CaF₂ singlecrystal lens R by adding 1400 mg of sodium CMC to the diamond slurry,and this adsorbing layer prevented or suppressed the reaction betweenthe CeO₂ single crystal lens R and the solvent of the diamond slurry.

As mentioned above, the reaction between the ionic material to bepolished and the polishing slurry can be prevented or suppressed byincorporating the nonionic adsorbing layer-forming dispersant such asthe ionionic water-soluble synthetic polymer as the dispersant into thepolishing slurry according to the present invention. Thereby, thesurface roughness and the surface defects can be reduced as comparedwith the conventional polishing slurries.

In addition, according to the polishing slurry of the present invention,the nonionic adsorbing layer is formed on the surface of the ionicmaterial by adding the nonionic adsorbing layer-forming dispersant suchas the ionionic water-soluble synthetic polymer thereto. Since thispolymer adsorbing layer prevents the reaction between the ionic materialto be polished and the polishing slurry, fine powders of nonionic-bondmaterials to be polished need not be prepared as in case of theconventional polishing slurries in which the fine powder of the ionicbond materials is dissolved to prevent the above reaction.

According to the claimed method for selecting the dispersant to be addedinto the polishing slurry, the nonionic adsorbing layer-formingdispersant such as the nonionic water-soluble synthetic polymer isselected based on the proceeded degree of the etching on the ionicmaterial. Therefore, when the thus selected dispersant is incorporatedinto the polishing slurry, the reaction between the ionic material andthe polishing slurry can be effectively prevented or suppressed.

According to the claimed method for determining the mixing concentrationof the dispersant to be added to the polishing slurry in the presentinvention, the mixing concentration is set depending upon the proceededdegree of etching on the ionic material. Therefore, when the polishingslurry having the thus set mixing concentration of the dispersant isused, the reaction between the ionic material and the polishing slurrycan be appropriately prevented or suppressed.

According to the polishing method with use of this polishing slurry, theoptical elements having high precision, excellent surface smoothness andno defects can be obtained. Such optical elements are favorably used inthe deep ultraviolet range.

Thus, the polishing slurry of the present invention can be used forpolishing the ionic materials including the fluoride crystals, and canattain excellent polished characteristics such as reduction in thesurface roughness and the surface defects.

In the above Embodiments, the nonionic adsorbing layer-formingdispersant such as the ionionic water-soluble synthetic polymer: sodiumCMC is selected as the dispersant, but the invention is not limitedthereto so long as the reaction between the ionic material and thepolishing slurry can be prevented or suppressed.

1. A polishing slurry to be used for polishing an ionic material, saidpolishing slurry comprising a dispersant which is to form a nonionicadsorbing layer on a surface of the ionic material.
 2. The polishingslurry claimed in claim 1, wherein said dispersant is a nonionicwater-soluble synthetic polymer.
 3. The polishing slurry claimed inclaim 2, wherein the nonionic water-soluble synthetic polymer is sodiumcarboxylmethyl cellulose (CMC).
 4. A dispersant-selecting method, whichis adapted to select the dispersant to be incorporated into thepolishing slurry claimed in claim 1, comprises separately preparing afirst solution containing a first dispersant and a second solutioncontaining a second dispersant different from the first one, immersingtest pieces each made of said ionic material into the first and secondsolutions, respectively, while a portion of each of the test pieces ismasked or not immersed, comparing a step between an etched portion and anon-etched portion of the test piece immersed in the first solution witha step between an etched portion and a non-etched portion of the testpiece immersed in the second solution, and selecting the dispersant usedin the solution in which the test piece having the smaller step isimmersed.
 5. A dispersant-selecting method, which is adapted to selectthe dispersant to be incorporated into the polishing slurry claimed inclaim 1, comprises separately preparing a first solution containing afirst dispersant and a second solution containing a second dispersantdifferent from the first one, immersing test pieces each made of saidionic material into the first and second solutions, respectively,comparing an average size of pits formed on the test piece immersed inthe first solution by etching with an average size of pits formed on thetest piece immersed in the second solution by etching, and selecting thedispersant used in the solution in which the test piece having thesmaller pit size is immersed.
 6. A method for determining a mixingconcentration of the dispersant selected by the selecting method inclaim 4, which sets the mixing concentration of the above-selecteddispersant to be incorporated into the claimed polishing slurry,comprises preparing a plurality of solutions having differentconcentrations of the dispersant, respectively, immersing test piecesmade of an ionic material into the plurality of the solutions,respectively, while a portion of each of the test pieces is masked ornot immersed, determining a relation between a step between an etchedportion and a non-etched portion of each of the test pieces immersed inthe plurality of the solutions, respectively, and the concentration of acorresponding one of the solutions, and setting the mixing concentrationof the dispersant to be used in actual polishing based on the thusdetermined relation.
 7. A method for determining a mixing concentrationof the dispersant selected by the selecting method in claim 5, whichsets the mixing concentration of the above-selected dispersant to beincorporated into the claimed polishing slurry, comprises preparing aplurality of solutions having different concentrations of thedispersant, respectively, immersing test pieces made of an ionicmaterial into the plurality of the solutions, respectively, while aportion of each of the test pieces is masked, determining a relationbetween a step between an etched portion and a non-etched portion ofeach of the test pieces immersed in the plurality of the solutions,respectively, and the concentration of a corresponding one of thesolutions, and setting the mixing concentration of the dispersant to beused in actual polishing based on the thus determined relation.
 8. Apolishing method comprises polishing an ionic material with a polishingslurry comprising a dispersant which forms a nonionic adsorbing layer ona surface of the ionic material, the dispersant being selected by aselecting method comprising separately preparing a first solutioncontaining a first dispersant and a second solution containing a seconddispersant different from the first one, immersing test pieces each madeof said ionic material into the first and second solutions,respectively, while a portion of each of the test pieces is masked ornot immersed, comparing a step between an etched portion and anon-etched portion of the test piece immersed in the first solution witha step between an etched portion and a non-etched portion of the testpiece immersed in the second solution, and selecting the dispersant usedin the solution in which the test piece having the smaller step isimmersed, at a mixing concentration determined by the mixingconcentration-determining method in claim
 6. 9. The polishing methodcomprises polishing an ionic material with a polishing slurry comprisinga dispersant which is a nonionic water-soluble polymer and which forms anonionic adsorbing layer on a surface of the ionic material, thedispersant being selected by a selecting method comprising separatelypreparing a first solution containing a first dispersant and a secondsolution containing a second dispersant different from the first one,immersing test pieces each made of said ionic material into the firstand second solutions, respectively, while a portion of each of the testpieces is masked or not immersed, comparing a step between an etchedportion and a non-etched portion of the test piece immersed in the firstsolution with a step between an etched portion and a non-etched portionof the test piece immersed in the second solution, and selecting thedispersant used in the solution in which the test piece having thesmaller step is immersed, at a mixing concentration determined by themixing concentration-determining method in claim
 6. 10. A polishingmethod comprises polishing an ionic material with a polishing slurrycomprising a dispersant which forms a nonionic adsorbing layer on asurface of the ionic material, the dispersant being selected by aselecting method comprising separately preparing a first solutioncontaining a first dispersant and a second solution containing a seconddispersant different from the first one, immersing test pieces each madeof said ionic material into the first and second solutions,respectively, comparing an average size of pits formed on the test pieceimmersed in the first solution by etching with an average size of pitsformed on the test piece immersed in the second solution by etching, andselecting the dispersant used in the solution in which the test piecehaving the smaller pit size is immersed, at a mixing concentrationdetermined by the mixing concentration-determining method in claim 6.11. The polishing method comprises polishing an ionic material with apolishing slurry comprising a dispersant which is a nonionicwater-soluble polymer and which forms a nonionic adsorbing layer on asurface of the ionic material, the dispersant being selected by aselecting method comprising separately preparing a first solutioncontaining a first dispersant and a second solution containing a seconddispersant different from the first one, immersing test pieces each madeof said ionic material into the first and second solutions,respectively, comparing an average size of pits formed on the test pieceimmersed in the first solution by etching with an average size of pitsformed on the test piece immersed in the second solution by etching, andselecting the dispersant used in the solution in which the test piecehaving the smaller pit size is immersed, at a mixing concentrationdetermined by the mixing concentration-determining method in claim 6.